In an observing or photographing optical device, several means have been suggested for detecting whether a photographing or image forming optical system is in a focusing state for clearly forming the image of an object to be photographed on a predetermined image forming surface or not. However, in case the object to be photographed is dark or in case a dark photographing optical system is used, the detecting output will be small and the focusing detection will be substantially impossible. In order to eliminate this defect, a light projecting means has been provided for the focusing detection. However, the structure is complicated, costly and bulky.
Generally, in such optical observing devices and optical photographing devices as endoscopes, cameras and television cameras it is important to determine whether or not the image forming position of the optical system of the device is in a focusing state coinciding with the predetermined image forming position on a film surface so that an object to be photographed may be clearly observed or photographed. Therefore, in some cases, a means of detecting whether or not the optical system is in a focusing state (hereinafter a focusing detecting device) has been provided. In other cases, a focusing setting device has been provided wherein an optical system is moved by a detecting signal of a focusing detecting device so as to automatically set the optical system in a focusing state.
In a conventional focusing detecting device, use has been made of a photoelectric element with a photoelectric converting function to receive a light. However, in case an object to be photographed is dark or in case a dark photographing optical system is used, the detecting output of the photoelectric element will be so small that the focusing detection will be substantially impossible.
Japanese Patent Publication No. 19810/1974 describes a means of projecting a beam of a fixed shape toward an object to be photographed. However, as an optical system independent of a photographing lens system is used for the projecting part, such can not be applied to an endoscope or the like in which photographing must be made in one light path.
Japanese Patent Laid Open No. 128923/1981 describes a means in which a split prism is used as a detecting device. In this case, at least two upper and lower minute photoelectric elements are required. As a result, there are problems which arise, in case a precision above a fixed level is required, as many of the above mentioned photoelectric elements will have to be arranged, the circuit system for detecting whether focusing is made or not by comparing their output signals will be complicated and, particularly, the products low in the production number will be costly.
Further, Japanese Patent Laid Open No. 125713/1981 describes a focusing detecting device wherein lights are projected onto an object to be photographed by flickering a light source. In this case, output signals in the extinguished sections are deducted from the output signals in the lighted sections so that the influences of other external lights than of the above mentioned light source may be reduced. However, besides having a predetermined image forming surface as a film surface, positions conjugate with this image forming surface are provided in two places on both sides of the image forming surface so that a light projected onto an object to be photographed by a light projecting means arranged in one place may be received by a light receiving means arranged in the other place. Therefore, there have been problems that comparatively large spaces are required for the light projecting means and light receiving means. Thus, the formation of the optical system and circuit system for focusing detection are complicated and costly.